How to deal with existential threats

I have been recently listening to an audiobook version of Steven Pinker's Enlightenment Now.  As a result I have been thinking about what are the real risks of existential threats to humanity as well as the liberal western secular democracies which seem to be at the pinnacle of human achievement currently.  The point of this is to put together some thoughts on what can actually be done to mitigate the risks of existential threats.  First, I want to go through a number of these threats to try and define characteristics of the threats, create some kind of categorization and finally give some thoughts to how to proceed to combat those characteristics.

Conspiracy theories

In looking at ways that humanity could be wiped off the face of the Earth or at least reduced technologically to some primitive hunter-gatherer or simple agricultural state, there are many possible mechanisms that have been proposed to do this.  I will give a brief synopsis of a dozen or so in no particular order

Gray goo

The goo can vary between genetically engineered bacteria to nanobots to even some kind of hypothetical alien being or technology that is introduced via space travel or comet (spaceship?) impact with the Earth.  In any case, there is something that simply converts whatever it touches into goo, removing the life and organization from the matter, destroying everything it comes in contact with.  A nice fictionalized version of this is Vonnegut's Cat's Cradle with the substance Ice Nine.

It came from space

There have been asteroid and comet impacts before and doubtlessly again in the future.  Comet impacts, but also other spatial hazards such as the sun going out, radiation, etc. fall into this category including missiles launched by hypothetical aliens.  Another space type hazard is the scenario where a particle accelerator creates a black hole and it then pulls in the entirety of the planet, the moon and eventually the entire sun.

Conquest

Some external force takes over from humans and in the process makes us go extinct.  Some possible sources are apes, high mortality infectious disease or hypothetical aliens or even zombies.

Changes to the environment

This includes all kinds of changes to the environment so that it is too hot, too cold, too dry, too acidic, too basic, too salty, too much radiation to sustain life anymore.  There are all kinds of reasons why there could be changes, some related to human activity, and others not.

Classification of the threat

I would propose classifying the threats on a few scales.  The first is the speed of arrival.  Since nothing can move faster than the speed of light, that would be the top of the scale.  Some of the astronomical or physics based existential threats do move at the speed of light.  But many threats are slower such as the sun going out.
The second scale would be the extent of human control there is on the threat.  Some threats such as asteroids and comets are outside our control.  Other threats such as nuclear war definitely are.
A third scale would be how between when it starts and annihilation.  For example, a supernova (not that our sun would ever supernova as it is too small) is over very quickly, but infectious disease might take a decade before it manages to kill everyone.
The kinds of existential threats that are easiest to handle are those that are entirely within our control, arrive slowly (can be seen years ahead) and take time to happen (allows for reaction).  We need to try and work towards shifting these scales for all threats in those directions, if possible.  For example, better detection of asteroids and comets, better control of research into new infectious diseases.  Our best tools to do this are evidence based science.